Re: [PATCH v1] perf record: collect user registers set jointly with dwarf stacks

[Arnaldo Carvalho de Melo]

Em Wed, Apr 17, 2019 at 08:45:08PM +0300, Alexey Budankov escreveu:
> Hi Arnaldo,
> 
> On 17.04.2019 18:48, Arnaldo Carvalho de Melo wrote:
> > On April 17, 2019 11:40:02 AM GMT-03:00, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> >> On Wed, Apr 17, 2019 at 11:35:42AM -0300, Arnaldo Carvalho de Melo
> >> wrote:
> >>> Em Wed, Apr 17, 2019 at 09:39:52AM +0200, Jiri Olsa escreveu:
> >>>> On Mon, Apr 15, 2019 at 06:36:13PM +0300, Alexey Budankov wrote:
> >>>>>
> >>>>> When dwarf stacks are collected jointly with user specified
> >> register
> >>>>> set using --user-regs option like below the full register context
> >> is
> >>>>> still captured on a sample:
> >>>>>
> >>>>>   $ perf record -g --call-graph dwarf,1024 --user-regs=IP,SP,BP
> >> -- matrix.gcc.g.O3
> >>>>>
> >>>>>   188143843893585 0x6b48 [0x4f8]: PERF_RECORD_SAMPLE(IP, 0x4002):
> >> 23828/23828: 0x401236 period: 1363819 addr: 0x7ffedbdd51ac
> >>>>>   ... FP chain: nr:0
> >>>>>   ... user regs: mask 0xff0fff ABI 64-bit
> >>>>>   .... AX    0x53b
> >>>>>   .... BX    0x7ffedbdd3cc0
> >>>>>   .... CX    0xffffffff
> >>>>>   .... DX    0x33d3a
> >>>>>   .... SI    0x7f09b74c38d0
> >>>>>   .... DI    0x0
> >>>>>   .... BP    0x401260
> >>>>>   .... SP    0x7ffedbdd3cc0
> >>>>>   .... IP    0x401236
> >>>>>   .... FLAGS 0x20a
> >>>>>   .... CS    0x33
> >>>>>   .... SS    0x2b
> >>>>>   .... R8    0x7f09b74c3800
> >>>>>   .... R9    0x7f09b74c2da0
> >>>>>   .... R10   0xfffffffffffff3ce
> >>>>>   .... R11   0x246
> >>>>>   .... R12   0x401070
> >>>>>   .... R13   0x7ffedbdd5db0
> >>>>>   .... R14   0x0
> >>>>>   .... R15   0x0
> >>>>>   ... ustack: size 1024, offset 0xe0
> >>>>>    . data_src: 0x5080021
> >>>>>    ... thread: stack_test2.g.O:23828
> >>>>>    ...... dso: /root/abudanko/stacks/stack_test2.g.O3
> >>>>>
> >>>>> After applying the change suggested in the patch the sample data
> >> contain
> >>>>> only user specified register values:
> >>>>>
> >>>>>   $ perf record -g --call-graph dwarf,1024 --user-regs=IP,SP,BP
> >> -- matrix.gcc.g.03
> >>>>>
> >>>>>   188368474305373 0x5e40 [0x470]: PERF_RECORD_SAMPLE(IP, 0x4002):
> >> 23839/23839: 0x401236 period: 1260507 addr: 0x7ffd3d85e96c
> >>>>>   ... FP chain: nr:0
> >>>>>   ... user regs: mask 0x1c0 ABI 64-bit
> >>>>>   .... BP    0x401260
> >>>>>   .... SP    0x7ffd3d85cc20
> >>>>>   .... IP    0x401236
> >>>>>   ... ustack: size 1024, offset 0x58
> >>>>>    . data_src: 0x5080021
> >>>>>    ... thread: stack_test2.g.O:23839
> >>>>>    ...... dso: /root/abudanko/stacks/stack_test2.g.O3
> >>>>>
> >>>>> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
> >>>>
> >>>> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> >>>
> >>> So, there are registers that are needed to do the DWARF unwinding,
> >>> right? But at the same time, if the user says only some are needed,
> >> he
> >>> better know what they're doing and ask for at least the registers
> >> needed
> >>> for the unwinding process to be successfull, right?
> >>
> >> yep, that's how understand that
> > 
> > So we need to document that, stating that specifying a set of registers together with requesting DWARF callchains may break things.
> 
> Do you mean break callchains if omitting IP,SP,BP? 
> For example like this: 
> $ perf record -g --call-graph dwarf,1024 --user-regs=AX,BX,CX -- matrix.gcc.g.O3

Right, i.e. if you don't use --user-regs, then a set of registers will
be asked for by --call-graph dwarf, right? If you use both and specify a
subset that doesn't have some of the asked for --call-graph dwarf, what
happens?

- Arnaldo